MBG, an endogenous Na/K-ATPase (NKA) inhibitor, promotes natriuresis via inhibition of renotubular NKA, but may cause vasoconstriction via inhibition of the NKA in the vasculature. Recently we demonstrated that ANP, via cGMP/PKG-2-dependent phosphorylation of renal alpha-1 NKA, sensitizes renal sodium pumps to MBG and may potentiate natriuretic action of MBG. In the vasculature, on the opposite, ANP, via PKG-1 dependent mechanism, reduces NKA phosphorylation and may offset the excessive vasoconstriction induced by MBG. Since aging is associated with a down-regulation of cGMP/PKG signaling, we hypothesized that in aged rats, ANP would not potentiate renal effects of MBG and would not oppose vascular effects of MBG. In young (3 month old) and aged (24 months old) Sprague-Dawley rats, we compared systolic blood pressure (BP), natriuresis, NKA activity in renal medulla and in vascular sarcolemma, and levels of MBG and alpha-ANP following acute NaCl loading (20%, 2.5 ml/kg, intraperitoneally), and the in vitro interactions of MBG and a-ANP on the NKA. As compared to young rats, NaCl-loaded aged rats exhibited greater MBG response (1.420.20 vs. 5.960.89 and 1.630.21 vs. 3.020.47 pmoles/hr, P<0.01), greater BP elevation (31 vs. 14 mm Hg, P<0.01) and greater inhibition of NKA in aortae (32 vs. 2%; P<0.01), less natriuresis (2.00.1 vs. 0.170.02 and 2.90.1 vs. 0.230.03 mmol/kg/hr; P<0.01) and less inhibition of NKA in renal medulla (25 vs. 42 %; P<0.05) in the presence of comparable changes in alpha-ANP and cGMP levels. Levels of PKG-1 in aorta and PKG-2 in the kidney in aged rats were markedly reduced, while levels of PDE-V in the kidney were increased. In aged rats, 1 nmol/L alpha-ANP did not affect level of alpha-1 NKA phosphorylation in aortic sarcolemma and renal medulla. Accordingly, in aged animals low concentrations of alpha-ANP did not potentiate MBG-induced inhibition of NKA from renal medulla and did not reduce effect of MBG on the NKA from aortic sarcolemma. Our results demonstrate that in aged rats, down-regulation of cGMP/PKG dependent signaling underlies a shift in ANP modulation of the effect of MBG on renal and vascular sodium pump, which promotes salt-sensitivity.